490 THE MECHANICAL FUNCTIONS. 



cumstance Avhich could contribute to give it light- 

 ness has been sedulously provided. The general 

 size of birds is considerably smaller than quadru- 

 peds of corresponding habits. No where has Nature 

 attempted to endow a huge ponderous animal, like 

 the fabled Pegasus, with the power of flight. Great 

 condensation has been given to the osseous sub- 

 stance,* in order that the greatest degree of strength 

 might be procured with the same weight of solid 

 materials; and the mechanical advantage derived 

 from their being disposed in the circumference, 

 rather than in central masses, has been obtained to 

 the utmost extent The horny material, of which 

 the stems of the feathers are constructed, are, in 

 like manner, formed into hollow cylinders, which, 

 compared with their weight, are exceedingly strong. 

 A similar shape has been given to the cylindrical 

 bones, which are fashioned into tubes with dense 

 but thin sides : most of the other bones have like- 

 wise been made hollow ; and instead of their cavities 

 being filled with marrow, they contain only air.f 

 Thus the whole skeleton is rendered remarkably 

 light : that, for instance, of the Pelicanus otiocro- 

 tahis, or white Pelican, which is five feet in length, 

 was found by the Parisian Academicians to weigh 

 only twenty-three ounces, while the entire bird 

 weighed nearly tw enty-five pounds.| The cavities 



* Ossification not only proceeds more rapidly, but is also carried 

 to a greater extent in this class of animals than in any other ; as a 

 proof of which, the tendons, especially those of the muscles of the 

 legs, are frequently ossified. 



t In the Bat there is no provision of this kind for hghtening the 

 bones; and we find them containing marrow, as in other mammalia, 

 and not air. 



t The dry skeleton of the Carrion Crow (Corvus corone) weighs 

 only twenty-five grains. Jaquemin, An. Sc. Nat. seiie 2, ii. 278. 



